Abstract

Silcrete fabrics are classified into: GS (grain-supported), F (floating-skeletal grains 'floating' in matrix), M (matrix-skeletal grains < 5% of volume), and C (conglomeratic-detrital component containing pebbles). F and M fabric types are subdivided on the presence/absence of glaebules. Replacement by silica of a precursor matrix can account for the development of all four fabric types, but some GS and C fabric silcretes have formed by passive silica precipitation in packing voids. Glaebules and colloform features composed of anatase and iron oxide are common in weathering profile silcretes, but their origin is uncertain. Authigenic silica is present as megaquartz, microquartz, length-slow and length-fast chalcedony (predominantly in void-fills), cryptocrystalline silica and opal-CT. Diagnostic petrographic features may be used with geochemical data and sedimentary associations to interpret silcretes with respect to weathering profile types (formed in humid, low pH environments) or nonweathering profiles types (formed in high-pH ? semiarid to arid environments).--Modified journal abstract.